February 19, 2014

Parabolic troughs and dry-cooled towers deliver similar value for concentrating solar power (CSP) plants, despite different solar profiles, a new NREL report has found. The findings demonstrate increasing importance for systems with thermal energy storage. The report also demonstrates that multiple CSP technologies and plant configurations can be analyzed using traditional planning tools such as production cost models.

CSP with thermal energy storage is a unique source of renewable energy in that the solar thermal energy can be dispatched in a similar manner as conventional thermal generation to respond to changes in supply or demand. CSP uses the thermal energy of sunlight to generate electricity. Parabolic troughs and power towers both concentrate sunlight onto a heat-transfer liquid, which is used to drive a steam turbine. Unlike photovoltaic energy, CSP can generate electricity not just when the sun is shining, but also after sundown, because a CSP plant can be built with thermal energy storage, such as molten salt.

The study analyzed the solar-multiple parameter—which indicates the thermal rating of the solar field relative to the capacity of the power block—and found that lower solar multiples had the largest marginal system value and benefitted from storage to prevent wasting surplus energy.

"We analyzed various plant configurations and identified specific ones that provide significantly more value than has been found in previous analyses," said Jorgenson, an analyst and the lead author of the report. Interestingly, however, "For all solar multiples, we found only a small benefit to storage from six to nine hours of rated plant capacity and less benefit beyond nine hours."

The NREL report, funded through the Energy Department's Office of Energy Efficiency and Renewable Energy in support of its SunShot Initiative, provides valuable quantitative results in a Colorado test system, comparing the two CSP technologies with thermal energy storage and evaluating how the operational and capacity value varies with plant configuration.

NREL is currently undertaking a similar analysis looking at the value of multiple CSP configurations in California under an assumed 40% penetration of renewables within that state.

"For both conventional and renewable energy systems, low levelized cost of energy does not necessarily reflect these systems' total value to the grid," Jorgenson added. "So, providing tools that utilities and grid operators are familiar with can lead to more informed decision-making as greater levels of renewable energy penetrate the market."